This invention relates to an average power control circuit, and more particularly to a logic circuit including analog and digital elements for gating a constant average power to an electrical load, such as a radiant energy fuser in an electrophotocopy machine.
It is desirable to accurately control the power delivered to an electrically powered heating load, especially when the heating load is a radiant energy fuser in an electrophotocopy machine. Such a fuser operates in a warm up mode, an idle mode and a fusing mode. There are a number of devices controlling the fuser during each mode of operation. Such devices range from simple thermostatic control to complex systems including thermal sensors and electronic circuits.
A problem with power controls is the generation of spurious radiation or noise. Sophisticated copy machines often contain a number of electronic components and noise will adversely interfere with the performance of such components. Noise will be generated whenever an ac power source is switched to the fuser unless the instantaneous value of ac voltage is near zero. Accordingly, it is desirable and known in the art to apply or interrupt power to the fuser when the instantaneous values of voltage and current are changing polarity, i.e., at their zero cross-over point. See, for example, U.S. Pat. No. 3,878,358 where a zero cross-over controlled fuser power supply is discussed.
It is also desirable to maintain the power applied to the fuser at a constant average value. By doing so, a precise idle temperature and fusing temperature can be maintained so that uniformly fused copies can be produced. However, it is difficult to accomplish this desirable result because voltage supplied by a utility company is not constant, and varies under the influence of factors beyond the precise control needed for optimum fusing consistency.
It is still further desirable to switch an even number of positive and negative ac half cycles to a fuser because an excess number of positive or negative half cycles will result in a net dc voltage delivered to the fuser. A net dc voltage will cause saturation in transformers in the electrophotocopy machine and in the utility supply line. Saturated transformers may overheat and otherwise malfunction thereby adversely affecting the electrophotocopy machine itself as well as other equipment that draws its electrical power from the same source as the machine.
It is especially important to control the power delivered to a focused radiant energy fuser. Such devices typically comprise a source of radiant energy and a shaped reflector. The reflector focuses the radiation on a narrow line of focus that is transverse to the path of a copy sheet bearing unfused toner particles. Those particles are melted and thereby permanently fixed to the copy sheet as they pass the line of focus. Slight fluctuations in the average power delivered to such a fuser may cause banding, i.e., transverse strips of inconsistently fused particles on the copy sheet. Accordingly, it is desirable to keep the average power delivered to the fuser at a nearly constant level in order to minimize or eliminate banding.